A general framework of canonical quasinormal mode analysis for extreme nano-optics

Optical phenomena associated with extremely localized field often should be understood with the consideration of nonlocal and quantum effects, which pose a difficult hurdle to conceptualize the physics with a few governing modes. Here we first propose a generalized Lorentz model to describe these effects and then develop a procedure to calculate and orthonormalize the quasinormal modes (QNMs) of the system, establishing a general canonical QNM theory for extreme nano-optics. Concrete formulism for materials described by the quantum hydrodynamic model is provided. The theory enables for the first time analyzing optical responses in quantum tunneling regime with quasinormal modes.